Conventionally, some diesel engines are provided with selective reduction catalyst incorporated in an exhaust pipe for exhaust gas flow and having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen; a required amount of reducing agent is added upstream of the reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of the discharged NOx.
In a field of, for example, an industrial plant with flue-gas denitration, it has been well known that ammonia (NH3) is effectively used as reducing agent to reduce and depurate NOx. However, for automobiles, safety in running with ammonia itself on board is difficult to ensure, so that, in recent years, nontoxic urea water has been used as reducing agent for selective reduction catalyst.
More specifically, if the urea water is added to the exhaust gas upstream of the selective reduction catalyst, the urea water is decomposed in the exhaust gas into ammonia and carbon dioxide gas under a temperature condition of about 170° C. or more, and NOx in the exhaust gas is satisfactorily reduced and depurated by the ammonia on the catalyst.
In such use of the urea water as reducing agent, the urea water is fed from an on-board urea water tank through a urea water supply line to an upstream side of the selective reduction catalyst. This kind of urea water tank is provided with a breather hose which allows air in the tank to escape outside for pressure releasing during a water supply operation.
Generally used as the breather hose is a rubber hose made from, for example, ethylene propylene diene terpolymer (EPDM). The hose has an end connected to an upper portion of the urea water tank and has the other end opened and suspending downward, so that the tank is opened at the upper portion to atmosphere and is prevented from being invaded by outer foreign matters as much as possible.
There exists, for example, the following Patent Literature 1 as a prior art literature pertinent to this kind of breather hose for a urea water tank.